In the past, communication controllers have been incorporated into data processing systems at a subsystem level, where they have had to have their own separate processors, or have required a dedicated direct communications line. One example of such a communication controller is described in U.S. Pat. No. 3,842,405, the subject matter of which is incorporated herein by reference.
In this patent the communication controller serving the various communication lines includes a microcontrol memory, an arithmetic-logic unit and a number of hardware registers. This controller is connected to the central processing unit (CPU) by means of an input/output channel.
One problem of prior art communication controllers is that they tend to be complicated and expensive. This is especially the case if the controller has to serve a large number of communication lines having different speeds and transmission characteristics.